Integrated circuits may function at different operating voltages depending on the fabrication process used and the intended purpose of the integrated circuit. Legacy integrated circuit input-outputs (I/Os) typically operate at either about zero or about five volts, e.g., logic 0 and logic 1. However, the newer and smaller process technology integrated circuits typically operate at either about zero or about three volts, e.g., VDD of about 3.3 volts, because new high circuit density fabrication processes produce transistors that cannot operate safely above a 3.3 volt VDD.
When interfacing five volt legacy integrated circuit devices to the newer technology 3.3 volt devices, external voltage level shifters or open drain outputs may be used. The external voltage level shifters add expense, complexity and take up space on a printed circuit board. Traditional open drain outputs have operating performance limited by the pull-up resistor in combination with the inherent circuit capacitance of each open drain output.